1. Field of the Invention
This invention relates generally to apparatus for processing digital data and more particularly to a data frame synchronizer for return-link signal processing apparatus receiving a serial data stream from an airborne vehicle such as a spacecraft.
2. Description of Related Art
The rapid growth of chip fabrication densities has resulted in an extremely positive impact of telemetry data systems, particularly those of the National Aeronautics And Space Administration (NASA), the assignee of the present invention.
New data system implementations are currently becoming smaller, cheaper and more powerful due to the availability of higher integration components developed through improved very large scale integrated circuit (VLSI) fabrication processes. For ground telemetry systems, many of these components are the latest standard commercial microprocessors and solid-state memories developed for general purpose computing. Although general purpose components have improved telemetry data system implementations, even greater improvements are being obtained with the addition of components developed specifically for telemetry processing. This effort has led to a series of many different telemetry processing semiconductor components implemented in silicon and gallium arsenide. The high integration levels offered by these components have enabled the development of VLSI based systems that provide an order of magnitude improvement in performance cost and size heretofore unobtainable.
To make full use of today's available VLSI densities, a new series of VLSI application specific integrated circuit (ASIC) components for return link data processing has been developed using the widely adopted packet telemetry protocols recommended, for example, by the Consultive Committee for Space Data Systems (CCSDS).
In the past, telemetry formats tended to be unique for each application or mission. This mission-unique development cycle has led to very high costs for the acquisition and maintenance of data handling systems. To reduce these costs and to promote interoperability between ground processing elements, space data protocol standards outlined by the CCSDS have recently been adopted, and as a result, most future missions, particularly those related to NASA, are being planned so as to use the aforementioned CCSDS protocols.
Systems implementing return link functions are used, for example, in ground stations, control centers, science data processing facilities, space verification equipment, compatibility testing and launch support facilities. Return link processing takes place after the acquisition, demodulation and digitalization of signals transmitted from a spacecraft, e.g. a satellite, as shown in FIG. 1.
Return link processing systems generally extract framed digital data from incoming signal bit streams, correct frame to frame data, validate the protocol structures within the frame, and extract user data. A typical return link processing chain for packetized CCSDS telemetry signals is shown in FIG. 4 and includes apparatus for providing frame synchronization, Reed-Solomon error correction, and service processing.
While the present invention is primarily directed to frame synchronization of telemetry data, it also provides frame synchronization of data from a weather satellite.
Frame synchronization is the process of demarcation of telemetry frame boundaries from an incoming serial bit stream by use of attached sync markers as shown in FIG. 3. Conventional CCSDS telemetry systems, moreover, use a specific digital sync marker pattern (ASM) such as shown in FIG. 3 to delineate data frame boundaries. Because space-to-ground transmission induces numerous types of data disturbances, frame synchronizers employ sophisticated measures in searching for these markers to ensure correct synchronization of data. Thereafter, Reed-Solomon error correction removes errors introduced during the transmission process while CCSDS processing demultiplexes, extracts and validates user data from the composite stream of telemetry frames.
Weather satellites transmit data in a pseudo noise (PN) pulse stream. Nevertheless, frame synchronization of such data is still required before any kind of data processing can be done.